1. Field of the Invention
The present invention relates to an organic electroluminescent display (ELD), and more particularly, to an organic ELD with high color purity with low driving current.
2. Discussion of the Related Art
Generally, organic ELD's use organic materials having high fluorescent or phosphorescent efficiency. Accordingly, band gaps of the organic materials may be easily developed through molecule design and synthesis. Moreover, organic ELDs can be fabricated on glass and plastic substrates because of their low fabricating temperatures.
FIG. 1 is a cross-sectional view of an organic ELD according to the related art. In FIG. 1, an organic ELD 10 includes a first electrode 12, a second electrode 14, and an organic layer 16 interposed therebetween. Electrons and holes are injected through the first and second electrodes 12 and 14, respectively. The organic layer 16 is composed of a hole transporting layer (HTL) 16a contacting the first electrode 12, an electron transporting layer (ETL) 16c contacting the second electrode 14, and an emitting material layer (EML) 16b interposed between the HTL 16a and the ETL 16c. The EML 16b is a electroluminescent organic layer that emits light by application of an electric field.
In the ELD, the electron and the hole injected through the first and second electrodes 12 and 14 combine into an exciton. When the exciton falls from an excited state to a ground state, light is emitted. Further, a ratio of “photons-out” per “charge-injected” increases, and a driving voltage decreases due to the HTL 16a and the ETL 16c. Since carriers are injected through a two step injection process using a transporting layer, the driving voltage may be reduced. Moreover, when the electron and the hole are injected into the EML 16b and move to an opposite electrode, a recombination may be adjusted since the electron and the hole are blocked by an opposite transporting layer. Accordingly, luminescent efficiency increases.
For a display device, the organic ELD 10 is formed on a large area substrate having a plurality of pixel regions that are composed of sub-pixel regions of red (R), green (G), and blue (B). Moreover, different organic materials are used within each sub-pixel region in order to display different colors.
FIG. 2 is a cross-sectional view of a full color organic ELD according to the related art. In FIG. 2, a passive matrix organic ELD includes a first electrode 32 formed on a substrate 30 along a first direction and a multi-layered organic layer 34 formed on the first electrode 32. A second electrode 36 is formed on the multi-layered organic layer 34 along a second direction crossing the first electrode 32, thereby defining pixel regions RP, GP and BP. The multi-layered organic layer 34 is composed of a hole induced layer (HIL) 34a, a HTL 34b, an EML 34c and an ETL 34d. The multi-layered organic layer 34 and the second electrode 36 are formed to extend along the second direction at each of the pixel regions RP, GP and BP. For the full color organic ELD, a different EML 34c is formed at each of the pixel regions RP, GP and BP.
FIG. 3 is a diagram showing an energy status of emitting material according to the related art. In FIG. 3, light is emitted when an exciton, which is generated by combination of an electron-hole pair individually injected from first and second electrodes, falls to a ground state. When the electron-hole pair have a spin (S) of S=½ combine into the exciton, a singlet exciton 20 having a spin of S=0 and a triplet exciton 22 having a spin of S=1 are generated with a ratio of 1 to 3. In the singlet exciton 20, two spins are inversely symmetric and in the triplet exciton 22, the two spins are symmetric. Even though the triplet exciton 22 has a lower energy than the singlet exciton 20 due to mutual interaction, a transition from the singlet exciton 20 to the triplet exciton 22 is basically prohibited because of spin changes. However, singlet exciton 20 can be substantially transitioned to the triplet exciton 22 due to spin-orbit coupling.
Since a ground state 24 of organic molecules is a singlet, a transition from the triplet exciton 22 to the ground state 24 of the singlet with emitting light is prohibited. Conversely, the singlet exciton 20 is transitioned to the ground state 24 of a singlet with fluorescence. However, the triplet exciton 22 can be substantially transited to the ground state 24 of a singlet with phosphorescence due to a perturbation, such as a spin-orbit coupling. Accordingly, in an ELD using fluorescent material, a triplet exciton does not contribute to total luminescence, whereas only a singlet exciton can contribute to the total luminescence. As a result, phosphorescent material using a triplet exciton is more energy efficient and has a longer material lifetime.
Conventional organic ELD's commonly use fluorescent material as an EML. However, the fluorescent material EML does not have sufficient luminescent efficiency. Specifically, since organic materials for R, G and B have different luminescent efficiencies, the EML of one color has lower luminescent efficiency than the EML of the other colors when driven with same current. Therefore, luminescent efficiency of an EML increases by raising a driving current for the EML so that adequate white purity of a full color organic ELD can be obtained. However, due to a high driving current, high power consumption is problematic.